1. Field of the Invention
The present invention relates to a hybrid envelope amplifier having improved efficiency, and more particularly, to an envelope amplifier using a dual switching amplifier and having improved efficiency in which power consumption is reduced by controlling a switching current of a switching region according to a magnitude of an envelope input signal, thereby improving efficiency compared to a conventional hybrid envelope amplifier.
2. Description of the Related Art
Efficiency of a power amplifier is an important factor that determines a use time of a battery. A portable terminal transmits a large volume of data such as a video as well as a voice data, and in order to transmit such a large volume of data, an orthogonal frequency-division multiplexing (OFDM) scheme which is a digital modulation scheme having high bandwidth efficiency and a peak-to-average-power-ratio (PAPR) is used.
A transmitter using this modulation scheme uses a linear amplifier to increase linearity. The linear amplifier may maintain linearity of a signal by operating in a back-off area to an extent of the PAPR at a maximum output power. The linear amplifier has maximum efficiency at the maximum output power; however, an output is rapidly decreased in the back-off area to degrade overall efficiency of the power amplifier. In order to solve this problem, an envelope tracking technology is used.
When an input signal is small, an envelope tracking power amplifier increases average efficiency by reducing a drain bias voltage. The drain bias voltage is obtained in a structure in which an envelope signal of an RF input signal is extracted and amplified through the envelope amplifier and the drain bias voltage is provided as an input to the power amplifier to adjust a supply voltage according to a magnitude of the RF signal. Efficiency of this structure is determined by the efficiency of the power amplifier multiplied by efficiency of the envelope amplifier, and thus, the envelope amplifier is one of very important components.
FIG. 1 shows an example of a transmitter provided with a conventional envelope tracking amplifier (hereinafter, interchangeably referred to as ‘envelope tracking power amplifier’ or ‘envelope amplifier’), FIG. 2 shows a conventional hybrid envelope tracking amplifier provided in such a transmitter, and FIG. 3 is a photo of a conventional hybrid envelope amplifier implemented in a monolithic microwave integrated circuit (MMIC) chip.
The transmitter provided with the conventional envelope tracking amplifier, as shown in FIG. 1, includes a digital signal generator, a multi-mode hybrid envelope amplifier, and a multi-band power amplifier, and an RF signal modulated and output through the digital signal generator is input to the amplifier envelope and the envelope input signal is used as an input of the envelope amplifier. The envelope tracking amplifier supports multi-modes while amplifying an envelope signal of a desired signal without distortion and maintaining high efficiency. An output of the envelope amplifier is used as the drain bias voltage of the power amplifier. When an input signal of the power amplifier is small, the drain bias voltage is decreased to achieve an effect of increasing overall efficiency.
In addition, the conventional envelope tracking amplifier, as shown in FIG. 2, has a structure in which the linear amplifier (linear stage) and a switching amplifier (switching stage) are combined. The linear amplifier operates as an independent voltage source which amplifies the envelope input signal, and the switching amplifier operates as a dependent current source which provides most of current needed at an output. The hybrid envelope amplifier implements high reliability in a linear region and implements high efficiency in a switching region, thereby satisfying both the reliability and the high efficiency.
However, although most of conventional envelope amplifiers have high efficiency, they are designed to be used in one mode, whereas a next-generation RF transmitter is required to be able to support both multi-bands and the multi-modes. In other words, the envelope amplifier as well as the power amplifier need to support the multi-modes; however, since the conventional envelope tracking amplifiers are designed to operate in one mode as described above, in order to support the multi-modes, amplifiers as many as a number of desired modes need to be connected in parallel and then power is applied to a desired amplifier for use when needed.
Therefore, the conventional envelope tracking amplifier and a transmitter provided therewith have a problem in that a chip and a module become larger, and are inadequate for a polar transmitter system for a single structure.
Accordingly, research results for satisfying various multi-band and multi-mode transmitter systems have been published. Among them, “High Efficiency Envelope Tracking Transmitter for 3G LTE Base Station Optimized with Inverse Class F (Class F−1) Amplifier and Two-bit Envelope Amplifier” (Non-Patent Reference 1), “A Polar Transmitter With CMOS Programmable Hysteretic-Controlled Hybrid Switching Supply Modulator for Multi standard Applications” (Non-Patent Reference 2), “Hybrid Envelope Elimination and Restoration Power Amplifier for 802.11g WLAN Applications” (Non-Patent Reference 3), “Circuits and System Design of RF Polar Transmitters Using Envelope-Tracking and SiGe Power Amplifiers for Mobile WiMAX” (Non-Patent Reference 4), “High Efficiency Envelope Tracking Power Amplifier with Very Low Quiescent Power for 20 MHz LTE” (Non-Patent Reference 5), “Apparatus and Method for Envelope Tracking Power Amplifier in Wireless Communication” (Patent Reference 1), and “A New Adaptive Power Amplifier Using Envelope Detection and Cross-talk Cancellation Technique and a Method of Controlling the Same” (Patent Reference 2) are included. In these conventional technologies, the envelope amplifier needs to have efficiency of about 80% or above for each mode in order to satisfy the multi-modes; however, efficiency of the conventional envelope amplifier is decreased according to an increase of a channel band width in each mode, and thus has a problem that the multi-modes are not supported.